This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device free from the defects caused by an induced domain.
Field effect liquid crystal display device of the twisted-nematic type is generally accepted to be suitable for use as the display device of a handheld calculator, for example, because of its low power consumption. This type of the display device is constructed and operates as follows. Thus, a liquid crystal having a positive dielectric anistropy is interposed between upper and lower parallel substrates made of transparent glass, for example, with the molecules of the liquid crystal paralleled with the upper and lower substrates and twisted 90.degree. therebetween. Such a display device is disposed between a pair of polarizers with polarizing axes intersecting with each other at right angles. With this construction, light impinging upon the device is polarized linearly by one of the polarizer. Then, its polarized plane is rotated 90.degree. by the liquid crystal molecules in a twisted arrangement and finally the light transmits through the other polarizer. Where transparent electrodes formed with a pattern of a letter, digits or the like are applied to the inner sides of the upper and lower substrates and impressed with a voltage of from several to several ten volts, the liquid crystal molecules will be arranged in substantially the vertical direction or in the direction of the field. Under these conditions, the polarized plane of the incident light will not be rotated by the liquid crystal molecules, thus the incident light will be intercepted. Thus, the pattern can be displayed by switching the operation between transmission and interception of the light.
With the prior art liquid crystal display device having a construction just described, a display defect called an optical activity domain occurs under no voltage impressed. More particularly, where the directions of the micro-groups of the molecules are arranged at right angles on the inner sides of the upper and lower substrates, the molecules twist in the clockwise and counterclockwise directions. In other words clockwise optical activity domains and counterclockwise optical activity domains coexist in the same device.
As a result, at the interface between two types of domains, the orientation of the molecules becomes random, thus diffusing the light with the result that the quality of the display is greatly degraded. Further, a defect termed an induced domain occurs under impressed voltage. More particularly, when the liquid crystal is subjected to electric field, the major axes of the molecules tend to orient in the direction of the field, but slightly incline with respect thereto.
Such inclinations occur with righthand end of the molecule raised and lefthand end thereof raised, thus forming regions having different directions of orientation depending upon respective states of inclinations. Under these conditions the contrasts of the display are differrent in respective regions thus resulting in a spotting display. This phenomenon is called an induced domain.
The optical activity domain described above, however, can be eliminated by controlling the directions of the micro-groups between the upper and lower substrates such that the angle of twist of the liquid crystal molecules differs slightly from 90.degree. or by incorporating into the liquid crystal such optical active substance as cholesteric. The induced domain is obviated by causing the liquid crystal molecules to orient slightly inclined, instead of parallel or horizontal, with respect to the upper and lower substrates under no voltage condition by providing orientation controlling structure formed, in general, by rubbing or obliquely vapor-depositing SiO substance. Such measure imparts polarities to the liquid crystal molecules so that the liquid crystal molecules orient in the same direction under electric field.
Where the direction of orientation is controlled in a manner described above, if the inclination of the molecules with respect to the upper and lower substrates is small, the induced domain still occurs especially at the edges of the transparent electrodes utilized in the display device. This tendency is enhanced when the display device is maintained at an elevated temperature.